This invention relates to improvements in vacuum processing of particulate reactive metal, such as in an electron beam or plasma furnace, and to an improved furnace for use in such processing.
Certain reactive metals such as titanium, for example, are prepared by reduction of chlorides of the metals using sodium or magnesium to produce sponge metal. Such sponge metals, however, contain trapped sodium or magnesium chloride and, when heated in a vacuum such as in an electron beam or plasma furnace, the trapped chlorides vaporize in an explosive manner, spraying unmelted sponge particles throughout the interior of the furnace so as to reduce the yield and also contaminate material which has been refined in the furnace with unrefined particles. Similarly, scrap material resulting from the machining or other forming of such metals which has been compacted into a solid piece for processing may contain vaporizable impurities which produce the same effect.
One way of avoiding this problem is to use an inert gas plasma burner which operates at higher pressures, as described in the Ulrich U.S. Pat. No. 3,771,585, but this does not provide the advantages of an electron beam or plasma furnace operated at high vacuum. The Hanks U.S. Pat. No. 3,101,515 discloses an electron beam furnace with magnetically guided beams in order to avoid contamination of the electron beam source by sponge particles explosively ejected from the raw material, but that arrangement does not avoid the problem of lost material and contamination of the refined material. The Herres U.S. Pat. No. 2,734,244 discloses a vacuum arc refining furnace for titanium sponge which requires a separate chamber to vaporize and drive off volatile inclusions from the sponge material which might interfere with the refining process, after which the material is delivered to the refining furnace.
In the copending Harker application Ser. No. 07/555,913, filed Jul. 19, 1990, such particulate material is compacted into bars which are conveyed toward the melting area of a hearth with end faces in opposed relation so as to intercept particles ejected from an opposing face and thereby block such material from reaching other parts of the vacuum furnace. That arrangement, however, not only necessitates compaction of particulate material into bar form, but also requires a complex and expensive bar-conveying system.